1. Field of the Invention
In general, the present invention relates to systems and methods that are used to illuminate an object for inspection by a camera system.
2. Prior Art Description
The largest problem associated with obtaining a clear image of an object or area is one of proper illumination. In order to obtain a clear and reliable image that is void of image artifacts from reflected ambient light, the object must be illuminated with light that is brighter than that of the ambient background light, else reflections and/or shade regions can obscure the details of the object being imaged. However, background full spectrum sunlight can sometimes exceed a brightness or irradiance of 100 mW/cm2. In order to overcome this level of ambient light, an object must be illuminated with a very intense flash. If the object being imaged is a person, or if a person is exposed to the flash, that person could be momentarily blinded by the intensity of the flash. Furthermore, a person would experience at least some physical discomfort from the light's intensity, especially by full spectrum white light outputted from a large aperture.
One solution that has been used to solve the problem of illumination is to illuminate objects with flashes of infrared light or deep blue light, instead of white light. The human eye is less sensitive to these bands of light as compared to the middle band wavelengths of visible light. In the prior art, infrared light is typically created with infrared LEDs, due to the commercial availability of these LEDs. However, a very large matrix of infrared LEDs would have to be used in order to surpass the infrared light contained in background sunlight. Furthermore, although the eye is far less sensitive to such infrared light, the use of infrared light greatly reduces the contrasts of the pattern being imaged as compared to broadband light. This is due to the fact that light produced by commercial LEDs is generally very narrowband, if not monochromatic. The small bandwidth of wavelengths being produced in combination with surface reflectance variances makes it more difficult to detect finely detailed patterns and texture in the image. Therefore, although the details of an object may be illuminated by LED light, the image obtained lacks much of the contrast detail needed for producing the richest image optimized for computer vision analysis.
Another problem associated with a large LED array that generates aggressive illumination is one of producing specularities. Specularities are the areas on an object that reflect the illuminated incident light back into the camera and cause an image saturation and obscuration of many details otherwise imaged. The reflected light appears as a white or a saturated area in the captured image, wherein no detailed information can be obtained. Physics governs that larger the solid angle of the illuminated light source area, the larger the specularity obscuration area that occurs within the image. Therefore, increasing the LED array to overcome ambient light intensity is counterproductive for maximizing the potential for capturing unobscured details.
The obvious solution to the above-identified problems is to eliminate background illumination and specularities by placing an object directly in front of a camera system in a light controlled environment. By enclosing an object in a light box, the object can be illuminated with highly diffuse light solving the many challenges of capturing unobscured, detailed images. This close-proximity imaging eliminates most ambient lighting problems and most specularities. Although controlled imaging may be appropriate for professional photographers at a photo shoot, such imaging techniques have little practical use when imaging objects and people in the real world where ambient light cannot be controlled. Nor is close proximity imaging practical for outdoor environments or naturally lit spaces where sunlight is prevalent and ever changing. Likewise, close proximity imaging has no applications in passive monitoring of moving objects, such as passing people, passing traffic or objects passing on an assembly line, where the object does not stop and face the camera to be imaged.
In order for a camera system to passively monitor a crowd, traffic or other moving objects including animals, the imaging camera must be focused at some preselected point. To avoid the complications, expense and reduced reliability of auto-focusing systems, the illumination system being used must be sufficient to illuminate the capture zone using a high F # lens that produces a sufficiently useful depth of field. The high level of illumination is compatible with the higher lens F # and produces a deeper static capture zone without an auto-focusing system. Different static capture zone designs are achieved by using different lenses designed to achieve various capture zone distances from several inches to beyond ten meters. The greater the static depth of field from the high F #, the more the auto-focusing system requirements are reduced, if not eliminated. It has proven difficult in the prior art to provide sufficient illumination throughout such an extended range without making the illumination flash either highly obvious or potentially harmful
A need therefore exists for a system and method, whereby a usable image can be obtained regardless of worst-case ambient lighting conditions. Furthermore, a need exists for a system that illuminates to optimize the fine details for imaging an object embedded with enhanced information created by the directional illumination that enables photometric stereo methodologies for rendering three-dimensional models in a manner that is not obvious, optically annoying, and/or potentially harmful. These needs are met by the present invention as described and claimed below.